1. Field of the Invention
The present invention relates to a motor drive, and more specifically relates to a motor drive having the function of protecting a dynamic braking circuit to be actuated in case of an emergency stop of a motor.
2. Description of Related Art
In motor drives for driving synchronous motors for feed axes of machine tools and industrial machines, industrial robots, and the like, dynamic braking circuits are widely used in inverter units, for the purpose of shortening stopping distances in case of emergency stops of the motors. The dynamic braking circuit includes resistors for converting rotational energy of the motor into heat, and switches for activating the circuit. The dynamic braking circuit is activated whenever an emergency stop occurs. The resistors used in the dynamic braking circuit have limited tolerances. However, when the driven motor has a high load inertia or is operated at a high speed, the rotational energy of the motor may exceed the tolerances of the resistors of the dynamic braking circuit. In such a case, it is necessary to reduce the rotational speed of the motor, or add a device to protect the dynamic braking circuit.
Various methods for protecting a dynamic braking circuit have been reported so far. For example, a temperature detector is provided in resistors of a dynamic braking circuit. Upon detecting extraordinary heat generation, the dynamic braking circuit is switched off (for example, Japanese Unexamined Patent Publication (Kokai) No. 2011-062035). According to this method, even in an abnormal condition, that is, in continued use of the dynamic braking circuit, the temperature detector detects the heat generation in the resistors of the dynamic braking circuit and interrupts a current flow through the resistors of the dynamic braking circuit. This serves to prevent damage to peripheral circuits due to the extraordinary heat generation. However, this conventional technique requires the addition of the temperature detector.
Also, a method in which consumed energy is calculated from two values, that is, the rotational speed of a motor at the time of starting application of a dynamic brake and an elapsed time between the start and the end of application of the dynamic brake, to determine whether or not to actuate a dynamic braking circuit is reported (for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-369564). This method allows easily calculating how much stored energy has been consumed in the dynamic braking circuit during the application of the dynamic brake, thus reliably performing a protection operation without adding any specific detector to the dynamic braking circuit. However, in this conventional technique, whether or not to actuate the dynamic brake can be determined only after a lapse of predetermined time from a time when the dynamic brake is intended to be actuated.
Also, a method in which in case of an emergency stop, power elements are turned on in one of upper and lower arms of an inverter, and turned off after the completion of switching of a dynamic braking circuit is reported (for example, Japanese Unexamined Patent Publication (Kokai) No. 2013-179741). This method allows an increase in the life of mechanical contact components that are used for shorting windings of a synchronous motor to apply a dynamic brake, a reduction in a braking time of the dynamic brake, and also the provision of a motor drive having a simple configuration at a low cost. Although this method aims at protecting contacts of switches of the dynamic braking circuit, this method does not consider the tolerances of resistors used in the dynamic braking circuit.